Method and device for detecting the contact of hands on a steering wheel

ABSTRACT

A method and a device are described for detecting the contact between hands and a steering wheel, which assures that the driver of a vehicle has his hands on the steering wheel.

FIELD OF THE INVENTION

[0001] The present invention relates to a method and device fordetecting the contact of hands on a steering wheel.

BACKGROUND INFORMATION

[0002] In connection with the present invention, the term “steeringwheel” is used synonymously with every type of steering mechanism. Thuscontrol sticks, such as are known, for example, in airplanes, also fallunder the concept of a steering wheel in the meaning of the presentinvention.

[0003] As the tracking of a vehicle becomes increasingly automated, itis necessary to determine whether the driver of the vehicle is carryingout his tasks or not. Thus, it has long been known, for example, fromthe railroad industry, using a so-called “dead man” switch, to check atregular intervals whether the train driver is awake or not. From thearea of motor vehicles, devices and methods are also known to regularlytest the reaction speed and therefore the wakefulness of the driver.From German Published Patent Application No. 195 18 914, a device isknown for testing the alertness and reactions of an automobile driver,which generates a signal that must be acknowledged by a finger pressureof the driver on the steering wheel. The time duration from theoccurrence of the signal to its acknowledgment is used as a measure forthe reactivity and alertness of the driver.

[0004] In the driver-assistance systems installed at least in futurevehicles, for example, tracking support or lateral wind compensation areto be provided. These functions can tempt the driver to take his or herhands from the steering wheel while driving. For this situation,driver-assistance systems are not provided. Rather, the driver shouldalways have control over the vehicle and should only be supported in asupplemental fashion by the driver-assistance systems. To assure thisdivision of labor between driver and driver-assistance system, thecontact of the hands of the driver on the steering wheel of the vehicleis continuously monitored.

[0005] The present invention is based on the objective of furnishing amethod and device for detecting the contact of hands on a steeringwheel, which is easy to assemble and which operates reliably over theentire service life of the vehicle.

[0006] This objective is achieved according to the present invention bya method for detecting the contact between hands and a steering wheel,in which a first signal having a first frequency is generated, the firstfrequency being a function of whether the hands of the vehicle driverare in contact with the steering wheel or not, a second signal having asecond frequency being generated, and an output signal being generatedas a function of the frequencies of the first and the second signal.

SUMMARY OF THE INVENTION

[0007] In the method according to the present invention, the hands ofthe vehicle driver cause a change in the frequency of a first signal, ifthey are in contact with the steering wheel. To increase the detectionrate, the frequency of the first signal is placed in relation to afrequency of a second signal in an appropriate manner, and an outputsignal is generated as a function of the frequencies of the first and ofthe second signal. The output signal provides information as to whetherthe hands of the vehicle driver are in contact with the steering wheelor not. As a result of the fact that the method according to the presentinvention operates without mechanical sensors, it is not subject towear. In addition, it is not impaired by subsequent modifications of thesteering wheel, for example, by a leather covering of the steeringwheel.

[0008] Further variants of the present invention provide for the outputsignal being generated as a function of the difference, or of the amountof the difference, between the first and the second frequencies, so thatit is possible in a simple manner to detect a frequency change that isgenerated by the contact of the hands on the steering wheel.

[0009] Further advantageous embodiments of the method according to thepresent invention provide that an output voltage be generated in afrequency/voltage converter as a function of the first and secondfrequency, and/or that the contact between the hands and the steeringwheel be detected if the output signal is greater than a thresholdvalue. As a result of these measures, the detection rates of the methodaccording to the present invention are further improved.

[0010] To compensate for changes in the first frequency due to externaldisturbance variables, a compensation can be provided. If the methodaccording to the present invention feeds a frequency into a firstresonant circuit having a capacitance, and the capacitance of theresonant circuit is changed by the contact between the hands and thesteering wheel, then, in a first refinement of the method according tothe present invention, changes in the first frequency due to externaldisturbance variables are compensated for.

[0011] The aforementioned objective is also achieved by device fordetecting the contact between hands and a steering wheel, having anarrangement for generating a first signal having a first frequency, thefirst frequency being a function of whether the hands are in contactwith a steering wheel or not, an arrangement for generating a secondsignal having a second frequency, and an arrangement for generating anoutput signal as a function of the first and the second frequencies.This device converts the method according to the present invention, sothat it also has the advantages inherent in the method.

[0012] In a further embodiment of the present invention, it can beprovided that the arrangement for generating a first signal at a firstfrequency include a first oscillator and a first capacitor connected inparallel with the former, and that the first capacitor is arranged onthe steering wheel. In particular, a steering wheel heating system canalso be a part of the first capacitor. By using a first oscillator and afirst capacitor, it is possible in a simple manner to detect whether thehands of the driver are on the steering wheel or not. If as a firstcapacitor the steering wheel heating system is used which is alreadypresent in luxury-class vehicles, it is possible to realize the deviceaccording to the present invention in the simplest manner and virtuallywithout additional expense.

[0013] In further embodiments of the device according to the presentinvention, the difference between the first and the second frequenciesis calculated in a mixer and/or is subsequently converted in afrequency-voltage converter into an output voltage proportional to theamount of the difference. If the output voltage is greater than aswitching threshold, then contact between the hands and the steeringwheel is detected.

[0014] The compensation for external disturbance variables provided infurther embodiments of the present invention can be effected throughadjusting the second frequency. When a complete compensation has takenplace, the first and the second frequencies are equal if the hands ofthe driver are not on the steering wheel.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 depicts a block diagram of an exemplary embodiment of adevice according to present invention.

[0016]FIG. 2 depicts a signal diagram of the device according to thepresent invention.

DETAILED DESCRIPTION

[0017] In FIG. 1, a block diagram is depicted of an exemplary embodimentof a device according to the present invention for detecting the contactbetween hands and steering wheel. In a first oscillator OSC 1, whichincludes a first capacitor C₁, a first signal having a frequency f₁ isgenerated. First capacitor C₁ is arranged on an undepicted steeringwheel of a likewise undepicted vehicle. It can be, for example, a partof an already present steering wheel heating system.

[0018] In a second oscillator OSC 2 having a second capacitor C₂ and anadjustable third capacitor C₃, a second signal having a second frequencyf₂ is generated. If the hands of the driver are not on the steeringwheel, first frequency f₂ and second frequency f₂ are identical. In amixer MIX, the amount of the difference between first and the secondfrequencies, f₁ and f₂, is calculated. The amount of the difference isconverted in a frequency-voltage converter 1 into an output voltage U.

[0019] If output voltage U is greater than a first switching thresholdS₁, the corresponding output signal S_(Output) is generated. This outputsignal is transmitted to an undepicted control unit of adriver-assistance system, which as a function of output signalS_(Output) activates or deactivates predetermined functionalities.

[0020] It has become evident that compensation for the changes of firstfrequency f₁ and second frequency f₂ due to external disturbancevariables can be omitted in most cases, because the differences in thedrift of first capacitor C₁ and second capacitor C₂ in the normal caseare virtually identical, and the difference between first frequency f₁and second frequency f₂ is evaluated. It is therefore advisable to placefirst oscillator OSC 1 and second oscillator OSC 2 at one or at twolocations in the vehicle that are exposed to the same environmentalinfluences. Furthermore, it is advantageous to install second capacitorC₂ in a well ventilated location on the steering wheel and not on aprinted circuit board of second oscillator OSC 2.

[0021] In the event that, despite these measures, compensation forexternal disturbance variables is still required, a static capacitancecompensation 3 can be provided. This can be done, for example, if theair humidity in the vehicle interior is subject to extreme changes, thusaltering first frequency f₁. The static capacitance compensation drivesthird capacitor C₃ of second oscillator OSC 2 such that first frequencyf₁ and second frequency f₂ are the same. This adjustment only occurs inresponse to slow changes in first frequency f₁, because the contactbetween hands and steering wheel is accomplished within fractions of asecond. Static capacitance compensation 3 receives from mixer MIX viafrequency/voltage converters 5 and 7 voltages U₁ and U₂, which are afunction of first frequency f₁ and second frequency f₂.

[0022] In order to be able to detect the speed of the changes of firstfrequency f₁ or of second frequency f₂, an arrangement is provided fordetecting alteration speed 9. This arrangement, as in the exemplaryembodiment according to FIG. 1, can be configured as a differentiator 9,which carries out the first temporal derivation of voltage U,hereinafter designated as U_(d). If voltage U_(d) is smaller than thirdthreshold value S₃, then the frequency change is a slow one, for whichthere is compensation.

[0023] Output voltage U is also supplied to static capacitancecompensation 3. As a result of the redundancy of the informationcontained in output voltage U as well as in voltage U₁ and U₂, thefunctional capacity of the device according to the present invention canbe at least partially tested.

[0024] On the basis of FIG. 2, three typical situations are described,and their effects on output signal S_(Output) are described.

[0025] Case 1: The Driver is Touching the Steering Wheel.

[0026] In time interval t₁ through t₃, the hands of the driver approachthe steering wheel and finally encircle it. As a result, first capacitorC₁ is influenced, and as a consequence thereof, first frequency f₁changes. The change in first capacitor C₁ as a result of the hands thatare situated on the steering wheel is depicted in FIG. 1 by the stylizedhand, capacitor C, and the dash-dotted lines from capacitor C to firstcapacitor C₁.

[0027] Because f₁ and f₂ are different in this case, behindfrequency-voltage converter 1 a voltage U arises, which is differentfrom 0 and which continuously increases during the approach of the handsto the steering wheel. As soon as first threshold value S₁ at time pointt₂ is exceeded, the contact between hands and steering wheel is detectedand output signal S_(Output) is different from 0. In time interval t₃through t₄, the driver has his hands on the steering wheel. In the timeinterval between t₄ and t₆, the driver releases his hands from thesteering wheel, which is reflected in a decreasing voltage U. As soon asvoltage U is smaller than first threshold value S₁, output signalS_(Output) (at time point t₅) returns to 0. It is detected that thedriver does not have his hands on the steering wheel.

[0028] Case 2: Capacitance Change as a Result of the Driver Entering theVehicle.

[0029] This situation is depicted in the time segment between t₇ andt₁₀. Between t₇ and t₈ as a result of the driver's entry, and between t₉and t₁₀ as a result of the driver's exit, the capacitance and thereforealso output voltage U do not change so sharply that voltage U exceedsfirst threshold value S₁. Therefore, output signal S_(Output) remains at0 also during the time period between t₇ and t₁₀. The rate of change ofvoltage U in time periods t₁ through t₃, t₄ through t₆, t₇ through t₈,and t₉ through t₁₀ are equal, so that voltage U_(d), which is calculatedin differentiator 9 from voltage U(t), is the same during each of thecited time periods. In the remaining time periods, voltage U does notchange, so that U_(d) is equal to 0.

[0030] Case 3: Slow Changes in Capacitance as a Result of ExternalDisturbance.

[0031] A change in capacitance of this kind, for example, as a result ofrain, takes place in time interval t₁₁ through t₁₂. Because this changein capacitance extends over a longer time interval dt and is also largeenough to exceed second threshold value S₂ of output voltage U and thirdthreshold value 3 of output voltage U_(d) of differentiator 9, thischange in capacitance is compensated for. This compensation takes placein time interval t₁₂ through t₁₃, yielding the result that outputvoltage U after t₁₃ is once again 0, because first frequency f₁ andsecond frequency f₂ are once again equal.

What is claimed is:
 1. A method for detecting a contact between hands ofa vehicle driver and a steering wheel, comprising: generating a firstsignal having a first frequency, the first frequency being a function ofwhether the hands are in contact with the steering wheel; generating asecond signal having a second frequency; and generating an output signalas a function of the first frequency and the second frequency.
 2. Themethod as recited in claim 1, wherein the output signal is generated asa function of a difference between the first frequency and the secondfrequency.
 3. The method as recited in claim 1, wherein the outputsignal is generated as a function of an absolute value of a differencebetween the first frequency and the second frequency.
 4. The method asrecited in claim 1, further comprising: causing a frequency-voltageconverter to generate an output voltage as a function of the firstfrequency and the second frequency.
 5. The method as recited in claim 1,further comprising: detecting the contact between the hands and thesteering wheel if the output signal is greater than a first thresholdvalue.
 6. The method as recited in claim 1, wherein a compensation ispresent for a change in the first frequency due to an externaldisturbance variable.
 7. The method as recited in claim 6, furthercomprising: feeding a frequency into a first resonant circuit having acapacitance, the capacitance of the first resonant circuit being changedby the contact between the hands and the steering wheel; andcompensating for changes in the first frequency due to the externaldisturbance variable.
 8. A device for detecting a contact between handsand a steering wheel, comprising: an arrangement for generating a firstsignal having a first frequency, the first frequency being a function ofwhether the hands are in contact with the steering wheel; an arrangementfor generating a second signal having a second frequency; and anarrangement for generating an output signal as a function of the firstfrequency and the second frequency.
 9. The device as recited in claim 8,wherein the arrangement for generating the first signal includes a firstoscillator and a first capacitor that is connected in parallel to thefirst oscillator, the first capacitor being arranged on the steeringwheel.
 10. The device as recited in claim 9, wherein a steering wheelheating system is a part of the first capacitor.
 11. The device asrecited in claim 10, wherein the steering wheel heating system can beswitched off for determining a capacitance.
 12. The device as recited inclaim 9, wherein the arrangement for generating the second signalincludes a second oscillator having a second capacitor.
 13. The deviceas recited in claim 8, wherein the arrangement for generating the firstsignal and the arrangement for generating the second signal are exposedto the same external condition.
 14. The device as recited in claim 12,wherein the first capacitor and the second capacitor are arranged in animmediate vicinity of each other.
 15. The device as recited in claim 8,wherein the arrangement for generating the second signal includes acapacitor that is connected in parallel to an oscillator.
 16. The deviceas recited in claim 8, further comprising: a mixer for calculating adifference between the first frequency and the second frequency.
 17. Thedevice as recited in claim 8, further comprising: a frequency-voltageconverter for converting a difference between the first frequency andthe second frequency into an output voltage that is proportional to anabsolute value of the difference.
 18. The device as recited in claim 17,wherein the contact between the hands and the steering wheel is detectedif the output signal is greater than a first switching threshold. 19.The device as recited in claim 15, wherein a compensation of the secondfrequency due to an external disturbance variable is present.
 20. Thedevice as recited in claim 19, wherein the compensation of the secondfrequency due to the external disturbance variable takes place byadjusting a third capacitor that is connected in parallel to theoscillator.
 21. The device as recited claim 8, wherein the firstfrequency and the second frequency are the same if the hands are not incontact with the steering wheel.
 22. A computer program that whenexecuted causes a performance of: generating a first signal having afirst frequency, the first frequency being a function of whether thehands are in contact with the steering wheel; generating a second signalhaving a second frequency; and generating an output signal as a functionof the first frequency and the second frequency.
 23. The computerprogram as recited in claim 22, wherein the computer program is storedon a storage medium.
 24. A control unit associated with a computerprogram that when executed causes a performance of the following:generating a first signal having a first frequency, the first frequencybeing a function of whether the hands are in contact with the steeringwheel; generating a second signal having a second frequency; andgenerating an output signal as a function of the first frequency and thesecond frequency.